Birches need shade, adequate water

It’s hard to beat the beautiful white bark of the European weeping birch tree. But as popular as they are, birch trees are not easy to grow in the Inland Northwest. A quick look through almost any neighborhood in Spokane will reveal birches with dead or dying tops and a generally unhealthy look.

In their natural habitat, birches live at the edge of the forest where their roots are shaded and cooled by other trees and their crowns have access to full sun. They are often found near water or wet areas where their fine roots receive adequate water.

Unfortunately, this is not where most homeowners plant them in a landscape. To show off the tree’s showy white bark and shape, people tend to plant them in the middle of the front lawn in full sun. As a result, instead of their roots being cooled by the shade of other trees, they are sitting out in the baking hot sun. The lawn planted around them hogs most of the water, leaving the sensitive roots subject to drought. The result is a stressed tree that begins dying back in a few short years, long before it should.

The final death knell for the tree usually comes as an attack by the bronze birch borer. The birch borer is a small, metallic bronze-colored beetle that attacks weakened or stressed birches. Scientists believe that birches under stress from drought or heavy pruning send out a physiological signal that draws the birch borer to the tree.

Adult beetles actively seek out stressed trees from May to August. The females lay eggs on the trunk, and on hatching the larvae bore into the tree and begin feeding on the cambium layer just under the bark surface. Eventually the larvae girdle the tree and cut off the tree’s ability to transport water and food, killing the tree.

Often the first sign a tree has been infested is stunted leaves and dying twigs at the top of the tree. To confirm the presence of the borers, look for worm-like trails under the bark and the presence of D-shaped holes in the trunk where the larvae emerged to go to their next target.

A tree will often die in stages. The first year the top will die, followed the next year by the next lower section or one member of a clump planting. Once a tree has suffered a dieback of about 25 percent, it is very difficult to save.

Birch borers can’t survive in healthy trees, so the best treatment is to plant it in a shaded area with adequate and even amounts of moisture. If a chemical treatment is necessary, the insecticide Sevin can be painted on the lower trunk of the tree before the adult females start laying eggs in May and then once a month through August, the flight period for the borer. This may prevent additional attacks but will not kill the larvae already under the bark.

Pat Munts is a Master Gardener in Spokane Valley. She can be reached at pat@inlandnw gardening.com.